The purchase is aimed at dramatically boosting Australia's ability to patrol 2.5 million square km (1 million square miles) of marine jurisdiction that include offshore oil and gas interests as well as important routes for energy and raw material supply.

While the order has long been expected, with Australia contributing to the development of the planes, it comes on the heels of China's first substantial military exercise in the eastern part of the Indian Ocean this month. That has sparked speculation that China too is showing greater interest in protecting its commercial sea lanes in the area.

More than 80 per cent of China's oil imports transit through the Indian Ocean on Australia's west coast. Japan, India and South Korea are also dependent on Indian Ocean routes. "The P-8A is a potent and highly versatile aircraft," Prime Minister Tony Abbott and Defence Minister David Johnston said in a joint statement. "As well as patrolling Australia's maritime approaches it can conduct search and rescue, anti-submarine and maritime strike missions using torpedoes and Harpoon missiles," the statement said.

The sale also provides an important shot in the arm for Boeing's defence division, where sales have been hard hit by a drop in US military spending.

The first P-8As will be delivered to the Royal Australian Air Force in 2017, with all eight aircraft fully operational by 2021. The order includes the cost of support facilities. Australia also has an option for a further four aircraft.

The planes are designed to replace the Royal Australian Air Force's AP-3C Orions, which have been in service for more than 40 years, and are expected to be supplemented with a fleet of MQ-4C Triton maritime surveillance superdrones at a cost of around $3 billion.

The Triton, under development by Northrop Grumman, is the size of a small airliner with a 40-metre (130-foot) wingspan. It can cruise at 20,000 metres (66,000 feet) for up to 30 hours, sweeping a distance greater than Sydney to London with 360-degree radar and sensors.

This would also provide surveillance compatibility with the US Navy, which plans to buy 68 Tritons when they enter service in late 2017.